1. Field of the Invention
This application is related to precision cleaning systems and, more particularly, apparatus for cleaning parts with supercritical fluids and applying sonic energy to the parts as a supplemental cleaning technique.
2. Description of the Prior Art
Today's manufacturing and assembly industries require parts which have a high degree of cleanliness. These requirements have led to development of an independent area of technology known as "precision cleaning". Precision cleaning may be defined as cleaning a given part to a degree that the level of foreign substances on the part meets a repeatably measurable standard. For example, parts which are to be chrome plated must be cleaned to a contaminant level of 20 micrograms per square centimeter, or less. Disc drive components for computers must be cleaned to a level less than 5 micrograms per square centimeter, and wafers utilized in the electronics industry must be cleaned to a level less than 1 microgram per square centimeter. In addition, there may also be a limit on the number of particulates of a certain size or larger which may be left on the part. For example, a typical specification may require that no more than 5,000 particles having a size greater than 2 microns should remain on the part. The various contaminants removed by precision cleaning include dissolvables, such as cutting fluid, particulates, such as diamond dust, and ionic bindings. Applications for precision cleaning include the manufacture of pens, razors and computer chips as well as various electronics industry applications.
The problem with presently available precision cleaning systems is that they use chlorofluorocarbons (CFC's) which are considered to destroy the earth's ozone layer. A system which utilizes CFC's is disclosed in U.S. Pat. No. 4,443,269 to Capella, et al. ("Capella"). Capella discloses a decontamination method for radioactive tools utilizing a high pressure spray gun for spraying the contaminated tools with freon. The general solution is to utilize more benign cleaning solvents, such as carbon dioxide. Carbon dioxide is particularly advantageous because it is a nonpolar solvent so that cosolvents may be added for a high degree of selectivity. It has been found that the cleaning capability of solvents such as carbon dioxide is enhanced when the solvent is raised to supercritical temperatures and pressures, or when supplemental cleaning techniques are utilized, such as sonic treatment.
The general concept of cleaning with supercritical fluids is known in the art. U.S. Pat. No. 5,013,366 to Jackson, et al. ("Jackson") discloses a cleaning process using phase shifting of dense phase gases. The solvent is shifted from its critical state to the liquid state and back by temperature adjustment while the solvent is in contact with the part to be cleaned. The cleaning apparatus utilized in Jackson is shown in FIG. 6.
Jackson schematically discloses and briefly discusses a cleaning vessel having an ultrasonic transducer in FIG. 8 and column 11, lines 36-50. However, Jackson does not teach or suggest the sonic arrangement according to the present invention. Nor does Jackson teach or suggest sonic application combined with mechanical agitation, which has been found particularly advantageous for removing sub-micron particulates.
It is an object of the present invention to provide an apparatus for applying sonic energy in combination with supercritical cleaning to enhance the cleanliness of the workpiece. It is a further object to add mechanical agitation in combination with sonic energy to assist in removing sub-micron particulates from the workpiece. It is a still further object to provide an apparatus which locates the workpiece, the sonic generation equipment and optional mechanical agitation equipment within a pressure vessel in such a way that supercritical cleaning fluid, sonic energy and mechanical agitation may be serially applied to the workpiece without the necessity of moving the workpiece from one station to another.